“The subject of voice calls on LTE networks has at times been utterly perplexing and incredulous – a mobile network that doesn’t natively support voice? What on earth were they thinking? The problem first emerged at the Mobile World Congress event a few years ago and was meet with general disbelief, yet it was true. Two solutions soon emerged, VoLTE and VoLGA (Voice over LTE Generic Access). The latter was created more as an interim solution, due to the frustration in getting VoLTE working.
VoLTE now looks like the winning option, with the backing of the majority of the LTE community. The problem is that without it, LTE operators still need the legacy 3G or 2G networks to carry voice traffic, as LTE is data only. The solution therefore is to simply convert voice into data packets and squirt this over the data network – except it hasn’t proved that simple.
That was the first problem. The next problem involves the handover of calls that pass from the still limited LTE network coverage and onto the wider 2G or 3G network. As Telefónica correctly explains in its release accompanying its technology test, while there has been recent progress with VoLTE technology in South Korea and the US, today’s LTE networks are not able to handover a VoLTE voice call to 2G or 3G when moving out of LTE coverage” TelecomTV